Conventionally, an OFDM modulation system is utilized for transmitting large capacity information in parallel. Such a system can transmit one type of data over a long time period, thereby reducing the effects of frequency selectivity phasing.
Further regarding an OFDM system, carrier intervals are shortened on a frequency axis, thereby improving frequency usability. Also, a copy signal of a symbol referred to as a guard interval is attached to the symbol. Therefore, it is possible to prevent symbols from interfering with one another when delayed radio waves are received.
Normally, the OFDM system rebuilds a sub-carrier signal, which includes errors introduced by frequency selectivity phasing, by error correction using other sub-carrier signal information. Accordingly, high precision error correction is required because transmitted data tends to delay spread. In such a case, however, several codes are required for error correction, thereby substantially decreasing signal transmission speed.
To avoid the above-mentioned decrease in signal transmission speed, conventional transceivers utilize an OFDM system with diversity reception. The diversity reception would enable the transceiver to select an appropriate signal from several signals received by several antennas. The transceiver can obtain a highest quality maximum-ratio-combining that maximizes the signal to noise ratio of respective sub-carrier signals.
Specifically, in the transceiver, sub-carrier information in the received signal can be rebuilt using identical sub-carrier signal received at the other antennas even if the received signal level of an antenna decreases. Accordingly, high quality information can be obtained without the need for high precision error correction.
An exemplary transceiver using an OFDM system with maximum ratio combining is described. Referring to FIGS. 7–9C, the transceiver has antenna elements 10–13, serial-parallel (S/P) converters 20–23, Fast Fourier Transformers (FFTs) 30–33, propagation path estimators 40–43, a weight calculator (maximum ratio combining weight calculators) 50, a maximum ratio combining processor 60, a demodulator 70 and a parallel-serial (P/S) converter 80.
Also, an example format of the OFDM signal is described. Referring to FIG. 8, a signal (timing signal) T that is for detecting signal timing and a signal (path signal) CS that is for estimating a propagation path are disposed before respective available symbols D1–DN (N is positive integer) on a frequency axis.
The timing signal T is a predetermined signal periodically transmitted in the time domain. The path signal CS has known sub-carrier signals arranged in a predetermined order in the time domain. The available symbols D1–DN include data symbols (Data (1)-Data (Nsym)) and guard intervals GI that are arranged before each of the data symbols. The data symbols D1–DN have frequency domain sub-carrier signals (data sub-carrier signals). The guard intervals GI are copies of data with respect to predetermined positions of respective data symbols D1–DN in which the guard intervals GI are provided. Incidentally, the available symbols are digital date modulated by a technique such as by BPSK, QRSK, 16 QAM or the like.
In FIG. 7, respective OFDM signals received by the antenna elements 10–13 are amplified and frequency converted by an RF/IF circuit (not shown). The resultant signals are input to a vector demodulator (not shown) and are demodulated to I and Q where I and Q correspond to real and imaginary numbers respectively. The I and Q are then processed by a synchronizing process, an AFC (Auto Frequency Control) process, a guard interval removing process or the like.
Further, respective resultant signals after the guard interval process are serial-parallel converted by the S/P converters 20–23. The parallel signals are then input into respective FFTs 30–33 which in turn generate demodulated OFDM signals.
In the OFDM demodulation, the FFTs 30–33 calculate respective known sub-carrier signals of the path signals CS and respective data sub-carrier signals of the available symbols because the OFDM signals include the path signal CS and available symbols D1–DN in the time domain.
Exemplary data sub-carrier signals calculated by the FFTs 30–33 are shown in FIGS. 9A–9C. In FIG. 9A, the horizontal axis shows frequency, and codes DS1–DS6 are respective data sub-carrier signals calculated by the FFTs 30–33. Phases and amplitudes of the data sub-carrier signals of the OFDM signals are different from each other even if the signals are simultaneously received. This is because the OFDM signals are deformed through the propagation path.
Accordingly, the data sub-carrier signals are compensated by the propagation path estimators 40–43 using, for example, the path signals DS1–DS6. The propagation path estimators 40–43 are provided for respective elements and replicate the known sub-carrier signals of the path signals CS. The propagation path estimators 40–43 complex-divide respective data sub-carrier signals, which are calculated by the FFTs 30–33, using the replicas of the known sub-carrier signals to thereby calculate propagation path estimating values of respective known sub-carrier signals that indicate frequency features of propagation path. In other words, the propagation path estimators 40–43 calculate the respective propagation path estimating values by dividing the received path signals CS by the respective replicas of the known sub-carrier signals.
The weight calculator 50 calculates the maximum ratio combining weight W using the propagation path estimating a value of respective sub-carrier signals. The weight W is shown in mathematical expression (1) below as a matrix of (number of the antenna elements)×(number of the data sub-carrier signals), where the propagation path estimating value is hi(l, k) whose antenna element number is “i”, available symbol number is “1” and data sub-carrier signal number is “k”.
                    W        =                  (                                                                                                                h                      1                                        ⁡                                          (                                              f                        ,                        1                                            )                                                                                                                                                                          h                          1                                                ⁡                                                  (                                                      f                            ,                            1                                                    )                                                                                                            2                                                                                                                                          h                      1                                        ⁡                                          (                                              f                        ,                        2                                            )                                                                                                                                                                          h                          1                                                ⁡                                                  (                                                      f                            ,                            2                                                    )                                                                                                            2                                                                              ⋯                                                                                                        h                      1                                        ⁡                                          (                                              f                        ,                        k                                            )                                                                                                                                                                          h                          1                                                ⁡                                                  (                                                      f                            ,                            k                                                    )                                                                                                            2                                                                                                                                                                  h                      2                                        ⁡                                          (                                              f                        ,                        1                                            )                                                                                                                                                                          h                          2                                                ⁡                                                  (                                                      f                            ,                            1                                                    )                                                                                                            2                                                                                                                                          h                      2                                        ⁡                                          (                                              f                        ,                        2                                            )                                                                                                                                                                          h                          2                                                ⁡                                                  (                                                      f                            ,                            2                                                    )                                                                                                            2                                                                              ⋯                                                                                                        h                      2                                        ⁡                                          (                                              f                        ,                        k                                            )                                                                                                                                                                          h                          2                                                ⁡                                                  (                                                      f                            ,                            k                                                    )                                                                                                            2                                                                                                      ⋮                                            ⋮                                                                                                                          ⋮                                                                                                                                h                      i                                        ⁡                                          (                                              f                        ,                        1                                            )                                                                                                                                                                          h                          i                                                ⁡                                                  (                                                      f                            ,                            1                                                    )                                                                                                            2                                                                                                                                          h                      i                                        ⁡                                          (                                              f                        ,                        2                                            )                                                                                                                                                                          h                          i                                                ⁡                                                  (                                                      f                            ,                            2                                                    )                                                                                                            2                                                                              ⋯                                                                                                        h                      i                                        ⁡                                          (                                              f                        ,                        k                                            )                                                                                                                                                                          h                          i                                                ⁡                                                  (                                                      f                            ,                            k                                                    )                                                                                                            2                                                                                )                                    (        1        )            
As shown in mathematical expression (1), respective elements of the weight W correspond to the respective data sub-carrier signals and are divided by an amount of the respective propagation path estimating values (square of the respective propagation path estimating values). The maximum ratio combining processor 60 maximum-ratio-combines the data sub-carrier signals using the weight W.
The resultant signal Z(l,k) generated by maximum-ratio-combining is shown in mathematical expression (2), where the sub-carrier signal is Xi(l,k) whose antenna element number is “i”, available symbol number is “1” and data sub-carrier signal number is “k”. Also, a total antenna element number is M and * means complex conjugate.
                              z          ⁡                      (                          ∫                              ,                k                                      )                          =                                            ∑                              i                =                1                            M                        ⁢                          hi              *                              (                                  f                  ,                  k                                )                            ⁢                              xi                ⁡                                  (                                      f                    ,                    k                                    )                                                                                        ∑                              i                =                1                            M                        ⁢                                                                            hi                  ⁡                                      (                                          f                      ,                      k                                        )                                                                              2                                                          (        2        )            
In the maximum-ratio-combining, the change of the identical data sub-carrier signals of the respective antenna elements 10–13 is compensated for using the propagation path estimating values. Thus, the maximum ratio combining processor 60 outputs combining signals z(l,k) with respect to each data sub-carrier signal.
The demodulator 70 digitally demodulates the combining signals z(l,k) using BPSK, QPSK, 16QAM or the like. Then, the P/S converter 80 converts the parallel demodulated signals z(l,k) to obtain serial demodulated data.
As mentioned above, the maximum ratio combining increases the signal levels of the received signals and decreases corresponding noise levels thereof. Namely, regarding the antennas 10–13, increasing the received signal level directs the main beam for reception toward a high signal energy direction. Also, decreasing the noise signal level decreases the side lobe levels. Accordingly, it is possible to direct the main beam for reception toward the target direction.
The above-mentioned receiver can direct the main beam for reception using the maximum ratio combining weight. That is, the reception beams of the antennas 10–13 are formed using the maximum ratio combining weight.
However, a transceiver that can direct a main beam for transmission in the same direction as that of the transmitter is not suggested.